This invention relates to cellular polyurethane materials having open cells and in particular to the use of certain cell opening agents to make these foams.
The production of foamed materials based on polyurethane and other polymer systems derived from organic polyisocyanates is well established. Depending upon the formulations used in their manufacture, the products can vary in texture from the soft flexible foams used as cushioning materials to the (semi-)rigid foams used as insulating or structural materials.
Whereas flexible foams are of the open cell type, (semi-)rigid foams can be either closed celled or open celled. An important use of open celled (semi-)rigid foam is as filler in evacuated insulation panels, as described, for example, in JP-A-133870/82, EP-A-498628, EP-A-188806. Another recently developed use of open celled (semi-)rigid foam is as filler for the core between the outer and inner walls of a thermal insulating device such as a refrigerator, a freezer or a boiler. To obtain good insulation properties the core is then afterwards evacuated and hermetically sealed. In order to maintain the required high vacuum it has been proposed to permanently connect the unit to a vacuum pump mounted in the unit itself (see WO 95/20136). Alternatively the foamed cavity after evacuation can be refilled with an insulating gas and then sealed, such as described in WO 97/21767.
Open celled (semi-)rigid polyurethane foam is made by reacting an organic polyisocyanate with a polyfunctional isocyanate-reactive composition in the presence of a blowing agent and a cell opening agent. Various cell opening techniques have been described in the past. These include the use of said cell opening agents such as polyethylene and polytetrafluoroethylene (as described, for example, in U.S. Pat. No. 5,312,846, U.S. Pat. No. 5,281,632 and U.S. Pat. No. 5,250,579), the use of silicone and non-silicone surfactants (as described, for example, in DE 3928867 and DE 3122790), the use of steam and the use of isocyanate-reactive cyclic urea's (as described, for example, in EP-A-498628 and EP-A-498629).
In most of the applications of open celled (semi-) rigid polyurethane foam a high open celled content (&gt;85%) is necessary. Particularly in the case of moulded foam (for example, when used to fill the core of a thermal insulating device) it has been proven very difficult with the existing cell opening techniques to obtain a high open celled content at high overpack and a minimum closed cell gradient (from the skin towards the core of the foam).
Therefore it is an object of the present invention to provide a new cell opening technique for (semi-)rigid polyurethane foams not showing the disadvantages mentioned above.